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Old 10-02-2007, 08:15 PM   #1
bdoss123
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Default 1997 GT - Steadily descreasing MPG , where to start?

Car has 129k on it.

Oil, Filter, and Air Filter changed about 1000 miles ago.

I read some posts after searching that indicated to check the fuel filter. What are other possible causes?
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Old 10-02-2007, 08:21 PM   #2
illegal2.2
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season change is also an issue. maybe try spark plugs, wires, pcv valve, etc..

choung
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Old 10-02-2007, 09:30 PM   #3
bdoss123
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How many miles should I expect out of the high dollar plugs? They were replaced at ~100k.

- BD
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Old 10-02-2007, 09:50 PM   #4
illegal2.2
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what kind was replaced at 100k? was it the stock platium ones?

choung
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Old 10-03-2007, 12:16 AM   #5
fibuz
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Quote:
Originally Posted by illegal2.2 View Post
what kind was replaced at 100k? was it the stock platium ones?

choung
copper*
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Old 10-03-2007, 04:18 AM   #6
BlockerImp
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coil packs, spark plugs are a great place to start
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Old 10-03-2007, 09:21 PM   #7
aross
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Front O2 Sensor.
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Old 10-04-2007, 01:31 AM   #8
coathegreatone
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definintely check the spark plugs and all the sensors
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Old 10-04-2007, 05:40 AM   #9
GrayT
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Agreed on the fuel filter... sounds like it got slightly clogged and is only getting worse.
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Old 10-04-2007, 02:31 PM   #10
linsavy
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I swapped the front o2 sensor even though there was no code and picked up 6mpg on the very next tank.
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Old 10-04-2007, 10:10 PM   #11
Jonathan
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Another remote possibility MIGHT be the Thermostat.

If it is stuck OPEN, it will take significantly longer for the motor to reach full operating temperature. This could actually prove even more significant in a cooler climate.

Also if you do a lot of "short" trips (say 5-10 miles or less) where the motor maybe isn't even reaching full operating temperature, the car will be running a significantly richer fuel mixture, which will significantly reduce fuel millage, and reduce the service life of both the oxygen sensor and the catalitic converter.
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Old 10-04-2007, 10:12 PM   #12
Jonathan
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NGK spark plugs are a good idea.

Denso, Bosch, and Champion are perhaps not a very good ideas.
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Old 10-05-2007, 09:30 AM   #13
omahasubaru
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- NGK copper plugs (I like V-power).
- New plug wires (I prefer OE)
- Fuel filter (I went with Purolator)
- PCV valve (I went with Purolator)

That's where I'd start. Then you could look at O2 sensors if you want, just get any cheap wire in sensor no need to spend high $$$ there.

The coil packs have had issues over the years on those engines with cracking, but it's usually evident by a miss at idle, so if your idle is fine... I'd be your pack is fine.


Any other issues w/ the car? Heres to hoping you don't have to do head gaskets anytime soon. Mine went at 135k. Even if they're leaking only enough to pressurize the coolant system, it can adversely affect your mileage. I was getting 22 mpg on mixed driving when the HG's were not in top shape. Now with a freshed top end and mild porting on the heads, but nothing else changed (stock exhaust/sensors) I'm getting 25-26 mpg mixed driving. which isn't bad for the EJ25D DOHC motors from what I've seen.

Jon
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Old 10-05-2007, 10:41 AM   #14
illegal2.2
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^ good info, as i can recall, i gotten lower mileage before my HG(headgasket) change also. bumped up a good 3-5mpg along with opening your exhaust and intake system like header back helped me 2-4mpg more.

choung
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Old 10-06-2007, 12:57 AM   #15
birukun
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I am at 122k, car started *drinking* gas, and now I have a CEL that says 'Bad Knock Sensor'.

Maybe yours is on its way out too.

Good luck-
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Old 10-06-2007, 08:19 PM   #16
Patrick Olsen
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Quote:
Originally Posted by bdoss123 View Post
Car has 129k on it.

Oil, Filter, and Air Filter changed about 1000 miles ago.

I read some posts after searching that indicated to check the fuel filter. What are other possible causes?
What was your mileage before, and what are you getting now? Same type of driving all along I assume? Where are you located? As Chuong said, it could just be a change in the gas formulation as we start heading towards winter. (With that said, I don't know when areas shift over.)

Quote:
Originally Posted by bdoss123 View Post
How many miles should I expect out of the high dollar plugs? They were replaced at ~100k.
What does "high dollar plugs" mean? Where they platinum plugs or copper plugs? I'm guessing platinum if they were "high dollar", which means they should be fine for a lot more miles on the car.

Quote:
Originally Posted by aross View Post
Front O2 Sensor.
Probably the most likely culprit in my mind, although new plug wires would be the first thing I'd take care of. In almost 191k miles of very hard use I've never had a coil pack go bad, so I'd be kinda surprised if that's the problem.

Quote:
Originally Posted by GrayT View Post
Agreed on the fuel filter... sounds like it got slightly clogged and is only getting worse.
I'm not sure I follow your logic here. A clogged filter would adversely effect performance if it got really bad, but I don't see how it would cause mileage to drop.

Quote:
Originally Posted by Jonathan View Post
Another remote possibility MIGHT be the Thermostat.

If it is stuck OPEN, it will take significantly longer for the motor to reach full operating temperature. This could actually prove even more significant in a cooler climate.

Also if you do a lot of "short" trips (say 5-10 miles or less) where the motor maybe isn't even reaching full operating temperature, the car will be running a significantly richer fuel mixture, which will significantly reduce fuel millage, and reduce the service life of both the oxygen sensor and the catalitic converter.
That could be, although as you said it's a remote possibility.

Quote:
Originally Posted by illegal2.2 View Post
^ good info, as i can recall, i gotten lower mileage before my HG(headgasket) change also. bumped up a good 3-5mpg along with opening your exhaust and intake system like header back helped me 2-4mpg more.
Good lord, what mileage are you getting now - 40mpg?? 5-9mpg better should be a good 50% improvement, which would indicate that your car was either in horrible shape before, or you're getting phenomenal mileage now!

Quote:
Originally Posted by birukun View Post
I am at 122k, car started *drinking* gas, and now I have a CEL that says 'Bad Knock Sensor'.

Maybe yours is on its way out too.
Another one that I don't really see the connection. I've had an over-active knock sensor before, which noticeably impacted performance and even day-to-day driveability, but mileage wasn't affected at all.

Pat Olsen
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Old 10-13-2007, 07:30 AM   #17
bdoss123
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Default

Wow, thanks for all the replies guys. I didn't check the thread until this morning.

Yes platinum plugs were installed around 100k, so I would hope they are fine.

Driving style/area has been the same, so I wouldn't think that would have changed my mileage. I'm in NC, bad mileage started about a month ago when the weather was still warm.

I used to get 23-25MPG and am now getting more like 19-22. It's only bothering me because my 1993 F250 Diesel is getting as good if not better mileage than the Sube.

Any way to tell if the HG's are going? I sure hope not, as this car probably isn't worth a whole lot more than that repair, lol.

Again, thanks for all of the replies.

- BD
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Old 10-14-2007, 09:43 PM   #18
calgearhead
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Quote:
Originally Posted by aross View Post
Front O2 Sensor.
Wouldn't it throw a CEL if going out? My mileage has dropped significantly (25 miles + to the tank) since my 800 mile journey on the highway, and could have sworn I had EVERYTHING above mentioned replaced LESS THAN 10k miles ago.
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Old 10-14-2007, 11:45 PM   #19
Patrick Olsen
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I actually had an O2 sensor die on me (verified by OBD2 datalogging) with no CEL.
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Old 10-15-2007, 03:26 PM   #20
emyselfandi
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Quote:
Originally Posted by birukun View Post
I am at 122k, car started *drinking* gas, and now I have a CEL that says 'Bad Knock Sensor'.

Maybe yours is on its way out too.

Good luck-
funny mine did the same at 125K... cheap and easy problem to fix though. Good threat. I've had reduced mileage since hitting 165K. I think it's b/c im in stop and go traffic and not rollin on the highway (moved into the city). The front 02 sensor sounds like good maint. though.
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Old 10-16-2007, 06:18 PM   #21
Radio Flyer
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I recently had a drop in mileage to 18 mpg from the usual 21. When I checked the air filter, noticed that there was a small hose disconnected from somewhere. After poking around for a bit, i found what looked to be an empty barb for a small hose (attached to a doohickey [technical term] on the strut tower and attached the hose to that.

Next tank, mileage was back to normal. The hose was completely hidden from view until I had lifted the filter cover.

Another possibility could be the gas. I had been buying gas from Sam's Club for years. And then my economy really started to tank. At 15mpg, I was starting to seriously consider selling my car. Following a friend's suggestion, I filled up with gas from Diamond Shamrock. Instantly, I was back to 21 mpg. Saving a nickel per gallon was costing me 4 gallons...

Just my 2 cents...
- Eric
Lil' Red '98 2.5 GT Wagon - Pics
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